Light dependent resistors, LDRs or photoresistors are often used in circuits where it is necessary to detect the presence or the level of light.

They can be described by a variety of names from light dependent resistor, LDR, photoresistor, or even photo cell, photocell or photoconductor.

Although other devices such as photodiodes or photo-transistor can also be used, LDRs or photoresistors are a particularly convenient electronics component to use. They provide large change in resistance for changes in light level.

In view of their low cost, ease of manufacture, and ease of use LDRs have been used in a variety of different applications. At one time LDRs were used in photographic light meters, and even now they are still used in a variety of applications where it is necessary to detect light levels.

Typical leaded light dependent resistor

What is light dependent resistor, LDR or photoresistor

A photoresistor or light dependent resistor is a component that is sensitive to light. When light falls upon it then the resistance changes. Values of the resistance of the LDR may change over many orders of magnitude the value of the resistance falling as the level of light increases.

It is not uncommon for the values of resistance of an LDR or photoresistor to be several megohms in darkness and then to fall to a few hundred ohms in bright light. With such a wide variation in resistance, LDRs are easy to use and there are many LDR circuits available. The sensitivity of light dependent resistors or photoresistors also varies with the wavelength of the incident light.

LDRs are made from semiconductor materials to enable them to have their light sensitive properties. Many materials can be used, but one popular material for these photoresistors is cadmium sulphide, CdS, although the use of these cells is now restricted in Europe because of environmental issues with the use of cadmium. Similarly cadmium CdSe is also restricted. Other materials that can be used include lead sulphide, PbS and indium antimonide, InSb.

Although a semiconductor material is used for these photoresistors, they are purely passive devices because they do not possess a PN junction, and this separates them from other photodetectors like photodiodes and phototransistors.

LDR / photoresistor symbol

The LDR symbol used in circuits is based around the resistor circuit symbol, but shows the light, in the form of arrows shining on it. In this way it follows the same convention used for photodiode and phototransistor circuit symbols where arrows are used to show the light falling on these components.

Photoresistor / light dependent resistor circuit symbols

The light dependent resistor / photoresistor circuit symbols are shown for both the newer style resistor symbol, i.e. a rectangular box and the older zig-zag line resistor circuit symbols.

How an LDR works

It is relatively easy to understand the basics of how an LDR works without delving into complicated explanations. It is first necessary to understand that an electrical current consists of the movement of electrons within a material.

Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. Insulators with a high resistance have very few free electrons, and therefore it is hard to make the them move and hence a current to flow.

An LDR or photoresistor is made any semiconductor material with a high resistance. It has a high resistance because there are very few electrons that are free and able to move - the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance.

As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice and some of their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance.

The process is progressive, and as more light shines on the LDR semiconductor, so more electrons are released to conduct electricity and the resistance falls further.

Photoresistor / LDR structure

Structurally the photoresistor is a light sensitive resistor that has a horizontal body that is exposed to light.

The basic format for a photoresistor is that shown below:

Photoresistor structure

The active semiconductor region is normally deposited onto a semi-insulating substrate and the active region is normally lightly doped.

In many discrete photoresistor devices, an interdigital pattern is used to increase the area of the photoresistor that is exposed to light. The pattern is cut in the metallisation on the surface of the active area and this lets the light through. The two metallise areas act as the two contacts for the resistor. This area has to be made relatively large because the resistance of the contact to the active area needs to be minimised.

This type of structure is widely used for many small photoresists or light dependent resistors that are seen. The interdigital pattern is quite recognisable.

The materials used for photoresists are semiconductors and include materials such as CdSe, CdS, CdTe, InSb, InP, PbS, PbSe, Ge, Is, GaAs. Each material gives different properties in terms of the wavelength of sensitivity, etc.

In view of the environmental concerns of using Cadmium, this material is not used for product in Europe.

Types of photoresistor

Light dependent resistors, LDRs or photoresistors fall into one of two types or categories:

Intrinsic photoresistors: Intrinsic photoresistors use un-doped semiconductor materials including silicon or germanium. Photons fall on the LDR excite electrons moving them from the valence band to the conduction band. As a result, these electrons are free to conduct electricity. The more light that falls on the device, the more electrons are liberated and the greater the level of conductivity, and this results in a lower level of resistance.

Extrinsic photoresistors: Extrinsic photoresistors are manufactured from semiconductor of materials doped with impurities. These impurities or dopants create a new energy band above the existing valence band. As a result, electrons need less energy to transfer to the conduction band because of the smaller energy gap.

Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light.

LDR frequency dependence

The sensitivity of photoresists is shown to vary with the wavelength of the light that is impacting the sensitive area of the device. The effect is very marked and it is found that if the wavelength is outside a given range then there is no noticeable effect.

Devies made from different materials respond differently to light of different wavelengths, and lots can be developed for the sensitivity and the light wavelength.

It is also found that extrinsic photoresists tend to be more sensitive to longer wavelength light and can be used for infrared. However when working with infrared, care must be taken to avoid heat build-up caused but he elating effect of the radiation.

Photoresistor / light dependent resistor latency

One important aspect associated with photoresists or light dependent resistors is that of the latency, or the time taken for the component to respond to any changes.

It takes a noticeable amount of time from any changes in light level before the LDR / photoresists attains its final value for the new level of light and for this reason the LDR / photo resistor is not a good choice where there are reasonably rapid changing values of light. However when the light changes take place over a period of time they are more than adequate.

The rate at which the resistance changes is called the resistance recovery rate. The LDR / photoresists normally responds within a few tens of milliseconds when light is applied after total darkness, but when light is removed it can take up to a second or so for the resistance to reach its final level.

It is for this reason that one of the specifications normally quoted in the photoresistor datasheets is the dark resistance after a given time, typically in seconds. Often two values are quoted, one for one second and another for five seconds. These given an indication of the latency of the resistor.

Photoresistor applications

Photoresistors are found in many different applications. They have a very simple structure and they are low cost and rugged devices.They are widely used in many different items of electronic equipment including photographic light meters, fire or smoke alarms as well as burglar alarms, and they also find uses as lighting controls for street lamps.

Extrinsic photoresists are provide sensitivity for longer wavelengths and as a result they are popular as info-red photodetectors. Photoresistors can also be used to detect nuclear radiation.

Light dependent resistor specifications

There are several specifications that are important for light dependent resistors, LDRs / photoresistors.

These photoresistor specifications include:

Key LDR / Photoresistor Specifications

Parameter

Details

Max power dissipation

This is the maximum power the device is able to dissipate within a given temperature range. Derating may be applicable above a certain temperature.

Maximum operating voltage

Particularly as the device is semiconductor based, the maximum operating voltage must be observed. This is typically specified at 0 lux, i.e. darkness.

Peak wavelength

This photoresistor specification details the wavelength of maximum sensitivity. Curves may be provided for the overall response in some instances. The wavelength is specified in nm

Resistance when illuminated

The resistance under illumination is a key specification is a key parameter for any photoresistor. Often a minimum and maximum resistance is given under certain light conditions, often 10 lux. A minimum and maximum vale may be given because of the spreads that are likely to be encountered. A 'fully on' condition may also be given under extreme lighting, e.g. 100lux.

Dark resistance

Dark resistance values will be given for the photoresistor. These may be specified after a given time because it takes a while for the resistance to fall as the charge carrier recombine - photoresistors are noted for their slow response times.

LDRs are very useful components that can be used for a variety of light sensing applications. As the LDR resistance varies over such a wide range, they are particularly useful, and there are many LDR circuits available beyond any shown here. In order to utilise these components, it is necessary to know something of how an LDR works, which has been explained above.